(1) Field of the Invention
The present invention generally relates to an apparatus for controlling a heater for an oxygen sensor, and more particularly to a heater control apparatus for varying electrical power supplied to a heater provided in an oxygen sensor fastened to an exhaust gas passage of an internal combustion engine so that the resistance value of the heater becomes equal to a target resistance value. The present invention is further concerned with a fuel control apparatus using such an apparatus for controlling the heater.
(2) Description of the Related Art
Recently, various control devices have been developed which are directed to an improvement in the output power in an internal combustion engine, a reduction of fuel consumption or a reduction of undesirable exhaust gases. Such control devices employ oxygen sensors. As is well known, an oxygen sensor can be used for measuring the concentration of an oxygen component contained in the exhaust gas. An oxygen sensor has a sensor element (sense portion) formed of a solid electrolyte or a semiconductor. An output signal of the oxygen sensor depends on the temperature of the sensor element thereof.
The oxygen sensor is equipped with a heater in order to make the sensor element operate effectively under a condition where the temperature of the exhaust gas is low and to stably maintain the temperature of the sensor element. Generally, the electric resistance of a metallic material is based on the temperature thereof. Thus, it is possible to maintain the metallic material at a constant temperature by maintaining the resistance thereof at a constant value. From the above point of view, the heater resistance of the heater is controlled by adjusting power supplied to the heater so that the heater resistance is maintained at a target resistance value (see Japanese Laid-Open Patent Publication No. 57-197459).
An internal combustion engine is controlled by a feedback control system using the oxygen sensor. A predetermined timing of fuel injection is adjusted based on the concentration of oxygen contained in, the exhaust gas detected by the oxygen sensor so that the mixture entering drawn into a cylinder has a target air-fuel ratio, such as a stoichiometric air-fuel ratio. In such a feedback control system, it is necessary to learn (calibrate) the heater resistance and correct for measurement error which are dependent on the electrical characteristics of the oxygen sensor.
For the above-mentioned purpose, Japanese Laid-Open Patent Publication No. 60-164240 discloses the following heater control apparatus. The temperature of inlet air and the temperature of a coolant are measured. It is recognized that the internal combustion engine is cooled when the inlet air temperature is almost the same as the coolant temperature. Since the heater of the oxygen sensor and the coolant are in a state of equilibrium, the temperature of the coolant can be considered to be the temperature of the heater of the oxygen sensor. The heater resistance obtained at 0.degree. K. is calculated, so that differences between the characteristics of the oxygen sensors can be compensated for.
The heater control apparatus disclosed in Japanese Laid-Open Patent Publication No. 60-164240 is based on the fact that the characteristics of different oxygen sensors vary differently with temperature. Referring to FIG. 1, lines I and II relate to two different oxygen sensors, and RH1 and RH2 denote heater resistance values obtained when the air-fuel ratio (A/F) is equal to a stoichiometric air-fuel ratio. It will be seen from the graph of FIG. 1 that the difference between the heater resistances given by the two lines I and II vary with sensor (heater) temperature. More specifically, the difference between the lines I and II increases as the sensor temperature increases. The above-mentioned conventional heater control apparatus does not take into account the fact shown in FIG. 1. Thus, the characteristic differences of oxygen sensors cannot be compensated for effectively, and an erroneous calibration of the heater resistance is carried out. As a result, the heater temperature is regulated at a temperature which deviates from the target temperature, and the air-fuel ratio deviates from the target air-fuel ratio. This degrades emissions.